Review ArticleUnravelling the eco-specificity and pathophysiological properties of Cutibacterium species in the light of recent taxonomic changes
Graphical abstract
Introduction
Cutibacterium (a cutaneous group of previously designated Propionibacterium) comprises one of the major genera of the microbiome of human skin and the eyes, and they are also well represented in the anaerobic component of human supra- and subgingival plaque and the intestinal tract [1]. The genus Cutibacterium arose out of the reclassification of genus Propionibacterium in 2016 into four genera, namely; Propionibacterium, Cutibacterium, Acidipropionibacterium, and Pseudopropionibacterium (later corrected to Arachnia), based upon habitat differences, biochemical tests and comparative genomics [2,3]. To date, five species are recognized under the genus; Cutibacterium acnes [2,4], Cutibacterium avidum [2,5], Cutibacterium granulosum [2,6], Cutibacterium namnetense [[7], [8], [9]], and Cutibacterium modestum (previously, “Propionibacterium humerusii”) [10] (Fig. 1). Since the reclassification in 2016, the new nomenclature is increasingly being used [[11], [12], [13], [14], [15], [16]]. All these species reside on the human skin as their major habitat. Previous reports indicate that C. acnes, the most predominant species of the group, is prevalent in sebaceous and dry areas of the skin, while the percentage is lower in moist areas [17]. Recently, three subspecies of Cutibacterium acnes were also established as C. acnes subspecies acnes (C. acnes type I) [[18], [19], [20]], subspecies defendens (C. acnes type II) [9,19], and subspecies elongatum (C. acnes type III) [18,20] (Fig. 1).
The taxonomy of these species and subspecies has been in a continuous state of flux (see summary, Table 1) and makes it difficult to reconcile clinical reports, which have not kept pace with changes in classification with nomenclatural changes. However, as interest in these taxa grew and new methodology was applied, taxonomic changes became inevitable. The most significant is the application of more in-depth proteomic and genomic analyses. As an example, the proposal of the subspecies of Cutibacterium acnes into Cutibacterium acnes subsp. acnes and Cutibacterium acnes subsp. elongatum were based not on traditional biochemical tests but on proteotyping methods and whole genome comparisons [18]. Because the species epithet Propionibacterium acnes has been established so long [21] whereas taxonomic changes have been so rapid and fairly recent, the old and new nomenclatures are often used interchangeably making it extremely difficult to ascertain possible correlations with specific infections. Consequently, the aim of this review is to clarify the taxonomic status of newly circumscribed taxa and draw parallels with the biology and pathogenic potential of various genotypes.
In this review, we commence with C. acnes, which is by far the best recognized species and most investigated, and then draw parallels with other Cutibacterium species to better understand the pathobiology of this group.
Section snippets
General description of the genus
Cutibacterium species are Gram positive, non-spore-forming, non-motile rods. Cells of many isolates are glass-shaped, but some isolates may be coccoid, filamentous, or V or Y-shaped [28]. Colonies on blood agar are lenticular, entire, smooth, white to light grey after 2–7 days of anaerobic culture. Growth is good under anaerobic and under microaerophilic conditions. Some isolates are aerotolerant, not only do they maintain viability under atmospheric air to grow later under anaerobic condition,
Conclusion
Phylogenetically based classification, especially whole genomes sequencing, circumscribes taxa into unambiguous taxonomic units that permit deeper insight into the biology of a species, especially its functional aspects in complex microbiomes. This is well demonstrated in the long history of the species first described as Bacillus acnes in 1891 and its later basonyms, Corynebacterium acnes and Propionibacterium acnes. Its morphological diphtheroid, coryneform rod-shaped and slightly curved
Funding source
None.
Declaration of competing interest
The authors declare that there is no conflict of interest.
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